1. Field of the Invention
This invention relates to a control apparatus for automatically slowing down a vehicle in order to suppress unstable movement of the posture of the vehicle while securing the traveling performance of the vehicle.
2. Description of the Related Art
In recent years, various techniques have been developed for controlling a vehicle to stabilize the posture or behavior of the vehicle.
One of such techniques is a yaw moment control technique wherein different braking forces are applied, for example, to left and right braking wheels to generate a moment in a turning round direction or a restoration direction (yaw direction) on the vehicle to control so as to enhance the steering characteristic (steering state) of the vehicle such as an understeer state or an oversteer state to correct the posture of the vehicle with respect to the turning direction to achieve stabilized traveling of the vehicle.
As another technique for controlling the posture of a vehicle upon turning, a roll-over suppressing control technique is available wherein braking force is applied to a turning outer wheel or wheels to suppress a motion (rolling motion) of the vehicle in an overturning direction. According to the roll-over suppressing control, when a vehicle turns, braking force is applied to a turning outer wheel or wheels to generate a yaw moment in a turning outer direction and simultaneously slow down the vehicle to prevent an increase of the roll rate or the lateral acceleration to be generated on the vehicle body to suppress a roll-over motion.
Meanwhile, since the simplest control for stabilizing the posture or the behavior of a vehicle is to decrease the vehicle speed, also an automatic slowdown control technique is known wherein braking wheels are braked to slow down the braking wheels to control the traveling speed of the vehicle until a vehicle speed at which the vehicle can turn safely is reached.
As one of automatic slowdown control techniques, a technique is known where in the turning radius of a vehicle, the coefficient of friction of the road surface on which the vehicle is traveling, and so forth are arithmetically operated based on the lateral acceleration, forward-backward acceleration (deceleration) and so forth generated on the vehicle and the traveling speed of the vehicle is controlled to a traveling speed at which the vehicle can turn safely. In the technique, for example, when a vehicle turns, the traveling speed of the vehicle is reduced so that the vehicle can keep a state wherein the wheels sufficiently grip the road surface.
When such slowdown control is carried out, it is significant how to set a start condition and an end condition of the control. In particular, the vehicle speed should coincide with the request for a speed of the driver, and the slowdown control should not be performed indiscriminately because it is control which is carried out against the request for a speed of the driver. Therefore, it is desired to decide with a high degree of accuracy whether or not the vehicle is in a state wherein the stabilization of the posture or the behavior of a vehicle should take precedence over the request for a vehicle of the driver and carry out the slowdown control only under a situation in which the slowdown control is required.
A technique directed to a start condition and an end condition of such slowdown control for achieving stabilization of the posture or the behavior of a vehicle is disclosed, for example, in Japanese Patent Laid-Open No. 2000-62499 (hereinafter referred to as Patent Document 1). According to the technique disclosed in Patent Document 1, as an end condition of the slowdown control, if behavior control of a vehicle is started although the vehicle is not in an unstable state, then the threshold value for an end of the behavior control is changed from that used when the behavior control is started thereby to end the started behavior control rapidly.
Another technique is disclosed in Japanese Patent No. 2,572,856 (hereinafter referred to as Patent Document 2). According to the technique disclosed in Patent Document 2, if an undesired turning behavior of a vehicle cannot be suppressed only by braking of a turning inner wheel or wheels, then it is decided that the vehicle speed is excessively high, and the excessive part of the vehicle speed is suppressed by automatic braking of turning inner and outer wheels so that an undesired turning behavior may not occur. Together with this, upon the automatic braking, the turning inner and outer wheels are braked in such a mode that the yaw rate of the vehicle may be a critical yaw rate which should be obtained in the suppressed vehicle speed condition.
Incidentally, the automatic slowdown control described above is directed to braking control by which the wheels can secure sufficient force of gripping of the road surface on which the vehicle travels. Thus, the automatic slowdown control is ended when the vehicle is slowed down to a speed at which the vehicle can travel sufficiently stably with respect to the magnitude of the turning radius. The start condition or the end condition of the control in this instance may be decided, for example, based on the lateral acceleration of the vehicle.
However, if the automatic slowdown control is ended with a fixed condition, then the automatic slowdown control may possibly be continued for more than a necessary period or may possibly be ended conversely although the automatic slowdown control is required further.
In particular, if the control end condition is set such that the automatic slowdown control is ended when, for example, the lateral acceleration of the vehicle drops to a predetermined value without taking the gradient of a road along which the vehicle is traveling during turning into consideration, then where the road is an uphill road, if the automatic slowdown control is carried out until after the lateral acceleration of the vehicle drops to the predetermined value, the slowdown occurs excessively due to the road gradient. This gives rise to a situation that, even if it is tried to accelerate the vehicle after the end of the control, sufficient acceleration cannot be achieved. On the contrary, where the road is a downhill road, if the automatic slowdown control is ended, for example, based on a decision that the lateral acceleration of the vehicle has decreased to the predetermined value, then the vehicle may thereafter be accelerated again by the road gradient, resulting in the possibility that the posture or the behavior of the vehicle may be rendered unstable again.
Patent documents 1 and 2 mentioned above do not disclose a solution to the subject just described.